Ion implantation is a physical process, as opposed to diffusion, which is a chemical process that is employed in semiconductor apparatus fabrication to selectively implant dopant into a semiconductor workpiece and/or wafer material. Thus, the act of implanting does not rely on a chemical interaction between a dopant and the semiconductor material. For ion implantation, dopant atoms/molecules are ionized and isolated, sometimes accelerated or decelerated, formed into a beam, and swept across a workpiece or wafer. The dopant ions physically bombard the workpiece, enter the surface and typically come to rest below the workpiece surface in the crystalline lattice structure thereof.
An ion implantation system is a collection of sophisticated subsystems, each performing a specific action on the dopant ions. Dopant elements, in gas or solid form, are positioned inside an ionization chamber and ionized by a suitable ionization process. In one exemplary process, the chamber is maintained at a low pressure (vacuum). A filament is located within the chamber and is heated to the point where electrons are created from the filament source, for example. The negatively charged electrons are attracted to an oppositely charged anode, also within the chamber. During the travel from the filament to the anode, the electrons collide with the dopant source elements (e.g., molecules or atoms) and create a host of positively charged ions.
Ion implantation using a broad ion beam has several advantages over other systems and methods based on a small or smaller sized beam, for example a pencil beam. Broad ion beams have low beam current density for better ion beam transport efficiency at low energy. They also provide simplicity in terms of mechanical scanning a uniform implant beam over the entire workpiece surface, no high acceleration/deceleration in workpiece motion and a simpler architecture in ion beam optics and mechanical wafer scanning system, to name a few. There have been numerous patents granted that show various approaches to produce broad beam and techniques to correct unavoidable intensity non-uniformity across the entire width of the beam. However, most known methods to correct beam intensity non-uniformity, sacrifice ion beam angle integrity for better uniformity (intensity).
The paper, “Positive Control of Uniformity in Ribbon Beams for Implantation of Flat-Panel Displays” describes typical uniformity correction methods which trade off decreased angle integrity versus better ion beam uniformity. (See e.g., White, N. R. Positive Control of Uniformity in Ribbon Beams for Implantation of Flat-Panel Displays, Institute of Electrical and Electronics Engineers, Inc. (IEEE), 1999, pp. 354-357).
Recent applications of ion implantation demand both intensity uniformity and angle consistency over the entire workpiece/wafer surface. The industry lacks an adequate system or method to control uniformity without effecting beam angle integrity and this inability to control the uniformity independently from other parameters has forced many implanter developers to shy away from broad beam technology.
Insertion of several metal or graphite rods into the ion beam to physically block a portion of the ion beam in order to control ion beam intensity uniformity was tried previously. Those rods can be arranged in a tight row to cover the entire width of the broad ion beam and the depth each rod is inserted into the beam can be controlled remotely. However, there are several problems associated with these techniques. First, the rods made of metal can cause generation of unwanted particles and metal contamination. Secondly, the slow response of the rods moved into and out of the ion beam makes controlling the beam difficult in terms of feedback control.
Accordingly, a suitable system and method for controlling the intensity uniformity of a broad ion beam are desired, that have a fast response time and do not sacrifice ion beam angle integrity.